1. Field of the Invention
The present invention relates to a switching amplifier that power-amplify and then outputs a quantized signal, like a PDM (Pulse Density Modulation) signal or a PWM (Pulse Width Modulation) signal, obtained by conversion from an analog signal or a one-bit signal.
2. Description of Related Art
A one-bit signal obtained by delta sigma modulation of an analog signal or a one-bit signal has the advantage of allowing the frequency characteristics thereof to be set to suit a sound source or the like through proper selection of the circuit constants of integrators and adders included in the delta sigma modulation circuit that performs the delta sigma modulation. This helps broaden the effective frequency range or the dynamic range. For this reason, new standards for CDs (compact discs) and SACDs (super-audio compact discs) adopt a one-bit signal for the recording of an audio signal; in practice, products conforming thereto have been commercially available. A one-bit signal is used not only for such recording of an audio signal but also for power amplification and for signal transmission between appliances.
A switching amplifier that power-amplify a one-bit signal obtained by delta sigma modulation feeds it, as it is, to the control terminal of a semiconductor power amplifier device so as to obtain high-voltage switching pulses based on the output of the semiconductor power amplifier device. Simply by passing these switching pulses through a low pass filter, it is also possible to obtain a power-amplified demodulated analog signal.
In addition, since the semiconductor power amplifier device is controlled with a one-bit signal obtained by delta sigma modulation, it operates in a nonlinear region (saturated region) thereof, rather than in a linear region (unsaturated region) as in an analog amplifier. Thus, a switching amplifier that power-amplifiers a one-bit signal obtained by delta sigma modulation has the advantage of being capable of highly efficient power amplification; in practice, products exploiting it have been commercially available.
An example of the electrical configuration of a conventional switching amplifier that power-amplifies a one-bit signal obtained by delta sigma modulation (see FIG. 7 of JP-A-2000-295049) is shown in FIG. 6.
The switching amplifier shown in FIG. 6 is composed of an input terminal 1, an adder 2, a delta sigma modulation circuit 3, a pulse amplifier 6 to which a constant voltage is applied by a constant-voltage power supply 7, a low pass filter 8, an output terminal 9 and an attenuator 10. The delta sigma modulation circuit 3 is composed of an integrator/adder group 4 and a quantizer 5. Here, the integrator/adder group 4 is provided with a plurality of integrators connected in cascade arrangement for integrating one portion after another of the signal inputted thereto, and an adder for adding up the outputs of the individual integrators. The quantizer 5 quantizes the signal outputted from the adder included in the integrator/adder group 4 to convert it into a one-bit signal.
An input signal (an analog signal or a one-bit signal) SIN inputted from an input signal source (unillustrated) via the input terminal 1 is supplied to the adder 2. A feedback signal SFB outputted from the attenuator 10 is also supplied to the adder 2. The adder 2 supplies the signal that is obtained by subtracting the feedback signal SFB from the input signal SIN to the delta sigma modulation circuit 3.
The delta sigma modulation circuit 3 converts the signal supplied from the adder 2 into a one-bit signal SQ, and sends the one-bit signal SQ to the pulse amplifier 6. The pulse amplifier 6 has a switching device such as a FET (unillustrated), and switches the switching device according to the one-bit signal SQ so as to power-amplify the one-bit signal SQ. The pulse amplifier 6 then sends the power-amplified one-bit signal to the low pass filter 8 and the attenuator 10. The output signal from the pulse amplifier 6 has the high-frequency components thereof eliminated by a low pass filter 8 to become an output signal SOUT, which is an analog signal. The output signal SOUT is then outputted via the output terminal 9. On the other hand, the output signal from the pulse amplifier 6 is attenuated by the attenuator 10 and thereby becomes the feedback signal SFB.
The pulse amplifier 6 that power-amplifies the one-bit signal SQ provides a balanced output by use of an H-bridge circuit as shown in FIG. 7. The switching device included in the pulse amplifier 6, even when it is a MOS type switching device, is formed by a process such as a Bi-CMOS or DMOS (Double diffused MOS) process so as to withstand high voltages and large currents. On the other hand, the delta sigma modulation circuit 3 provided in the stage preceding the pulse amplifier 6 is formed by a CMOS process. Thus, the pulse amplifier 6 and the delta sigma modulation circuit 3 are formed by different processes, and accordingly they are not integrated into the same semiconductor chip.
Thus, the distance from the delta sigma modulation circuit 3 to the pulse amplifier 6 or to a feedback section is rather long. The resulting parasitic capacitance, wiring resistance and the like across the signal path cause a blunting of the signal waveform and a signal delay across the signal path. Disadvantageously, this lowers the SN ratio of the output of the switching amplifier, and lowers the amplitude limit value of the input signal SIN to the delta sigma modulation circuit 3. Moreover, variations in the capacitances and impedances that determine the circuit constants of the integrators and adder included in the integrator/adder group 4 provided in the delta sigma modulation circuit 3, and also variations in the impedance of the wiring in the feedback section through which a signal based on the output signal from the pulse amplifier 6 is fed back to the delta sigma modulation circuit 3, cause variations in the pulse width of the output signal from the pulse amplifier 6.
In the pulse amplifier 6 that power-amplifies the one-bit signal SQ, as described above, a balanced output is provided by use of the H-bridge circuit as shown in FIG. 7. Here, the configuration adopted is such that the output is little affected by fluctuations in the supplied voltage V that is switched (the output voltage of the constant-voltage power supply 7). To reproduce an audio signal at a high output power, however, the H-bridge circuit as shown in FIG. 7 requires at least four very expensive amplifier devices that withstand high voltages and large currents. Furthermore, since the H-bridge circuit shown in FIG. 7 switches the direction of the output current so as to provide the balanced output, a dead time needs to be provided so as to prevent a through current from being generated when the direction of the output current is switched. Disadvantageously, this dead time causes distortion in the reproduced sound.
Moreover, in the switching amplifier shown in FIG. 6, in a case where the H-bridge circuit as shown in FIG. 7 is used in the pulse amplifier 6 that power-amplify the one-bit signal SQ, the one-bit signal SQ serving as a switching control signal needs to be somehow (for example, see JP-A-2002-208824) prevented from becoming indeterminate at the start of operation when the feedback loop, from the delta signal modulation circuit 3 to the pulse amplifier 6 to the attenuation circuit 10 and to the adder 2, is cut, because otherwise the switching device included in the pulse amplifier 6 breaks down.